1. Field of the Invention
The present invention relates to a method for fabricating a solar cell structure, particularly to a mixed-type heterojunction thin-film solar cell structure and a method for fabricating the same.
2. Description of the Related Art
Fossil fuels are going to be exhausted by the end of this century. The search and development for substitute energies, such as wind power, tide power, and biofuel, has been progressing for a period of time. Among them, solar energy has relative higher market acceptability, and many nations have been devoted to the development of solar energy. The German Advisory Council on Global Change predicted that solar energy will provide about 60% of the total energy in 2100. Solar energy is generated by the photovoltaic effect, wherein solar energy material directly converts sunlight into electric energy. The crystalline silicon solar cell has been developed for tens of years, and the related technologies thereof have been very mature. Generally, the monocrystalline silicon solar cell has an energy conversion efficiency of as high as about 20%. However, the fabrication cost thereof is too high to popularize solar energy. The topics of solar energy researches would be developing new materials, processes and systems to promote the energy conversion efficiency and reduce the cost of solar energy.
As to the polymer solar cell, the bi-layer solar energy element containing donors and acceptors was the one used at first. However, the contact area between donors and acceptors is too small to increase the probability of exciton (hole-electron pair) fission. Thus, the performance of this type of polymer solar cell is hard to promote. The solar cell containing the mixed donor material and acceptor material is called the BHJ (bulk heterojunction) cell, which has a greater donor-acceptor contact area than the bi-layer solar cell. Further, the spacing between donors and acceptors is within nanometers, which is near the exciton diffusion length—about 1-10 nm. Thus, the probability of exciton fission increases. The local electric field in the junction, which disjoins charges, originates from the HOMO (Highest Occupied Molecular Orbital) of donors and LUMO (Lowest Unoccupied Molecular Orbital) of acceptors. Therefore, more electrons and holes are disjoined at junctions. Thus is solved the problem of exciton diffusion length in polymer semiconductor. Then, the entire active layer is able to convert light into current. The mixture of donor material and acceptor material is spin-coated on a substrate to form an active layer. However, the two materials are not uniformly distributed in the active layer, which results in the following two cases. One is that there are likely to be paths directly connecting the anode and cathode, decreasing the parallel resistance and increasing leakage current. The other is that there are likely to be isolated areas having none path to the electrodes, wherefore the external circuit cannot collect the current of holes and electrons separated in the isolated areas.
Accordingly, the present invention proposes a mixed-type heterojunction thin-film solar cell structure and a method for fabricating the same to solve the abovementioned problems, whereby solar cells can possess flexibility and high energy conversion efficiency at the same time.